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The CMEMS Globcolour Chlorophyll a Product Based on Satellite Observation: Multi-Sensor Merging and flagging Strategies

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The CMEMS Globcolour Chlorophyll a Product Based on Satellite Observation: Multi-Sensor Merging and flagging Strategies Ocean Sci., 15, 819–830, 2019 https://doi.org/10.5194/os-15-819-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. The CMEMS GlobColour chlorophyll a product based on satellite observation: multi-sensor merging and flagging strategies Philippe Garnesson, Antoine Mangin, Odile Fanton d’Andon, Julien Demaria, and Marine Bretagnon ACRI-ST, Sophia Antipolis, 06904, France Correspondence: Philippe Garnesson ([email protected]) Received: 21 December 2018 – Discussion started: 2 January 2019 Revised: 20 May 2019 – Accepted: 21 May 2019 – Published: 24 June 2019 Abstract. This paper concerns the GlobColour-merged will require a better characterisation and additional inter- chlorophyll a products based on satellite observation (SeaW- comparison with in situ data. iFS, MERIS, MODIS, VIIRS and OLCI) and disseminated in the framework of the Copernicus Marine Environmental Monitoring Service (CMEMS). This work highlights the main advantages provided by the 1 Introduction Copernicus GlobColour processor which is used to serve CMEMS with a long time series from 1997 to present at The Copernicus Marine Environmental Service (CMEMS) the global level (4 km spatial resolution) and for the Atlantic provides regular and systematic reference information on the level 4 product (1 km spatial resolution). physical state and on marine ecosystems for global oceans To compute the merged chlorophyll a product, two major and for European regional seas, including temperature, cur- topics are discussed: rents, salinity, sea surface height, sea ice, marine optical properties and other such parameters. – The first of these topics is the strategy for merging This capacity encompasses satellite and in situ data- remote-sensing data, for which two options are consid- derived products, the description of the current situation ered. On the one hand, a merged chlorophyll a product (analysis), the prediction of the situation a few days ahead computed from a prior merging of the remote-sensing (forecast) and the provision of consistent retrospective data reflectance of a set of sensors. On the other hand, a records for recent years (re-analysis). merged chlorophyll a product resulting from a combi- The Ocean Thematic Assembly Centre (OCTAC) is part nation of chlorophyll a products computed for each sen- of CMEMS and is dedicated to the dissemination of ocean sor. colour (OC) products derived from satellite-based remote – The second topic is the flagging strategy used to discard sensing (Le Traon et al., 2015). OCTAC provides global and non-significant observations (e.g. clouds, high glint and regional (Arctic, Atlantic, Baltic Sea, Black Sea and Mediter- so on). ranean Sea) products for the period spanning from 1997 to the present. These topics are illustrated by comparing the CMEMS Glob- For global products, the Copernicus GlobColour proces- Colour products provided by ACRI-ST (Garnesson et al., sor has been used operationally since 2009 to serve CMEMS 2019) with the OC-CCI/C3S project (Sathyendranath et al., and its precursors (a series of EU research projects called 2018). While GlobColour merges chlorophyll a products MyOcean). with a specific flagging, the OC-CCI approach is based on The GlobColour processor was initially developed in the a prior reflectance merging before chlorophyll a derivation framework of the GlobColour project that started in 2005 as and uses a more constrained flagging approach.Although an ESA Data User Element (DUE) project to provide a con- this work addresses these two topics, it does not pretend tinuous data set of merged L3 ocean colour products. Since to provide a full comparison of the two data sets, which the beginning of the project, GlobColour has been continu- Published by Copernicus Publications on behalf of the European Geosciences Union. 820 P. Garnesson et al.: The CMEMS GlobColour chlorophyll a product Based on satellite observation ously used by more than 600 users worldwide. This effort has been continued in the framework of CMEMS to derive (among others) the chlorophyll a ocean colour core product. Many algorithms have been published to retrieve chloro- phyll a from reflectance data derived from satellite obser- vations (e.g. Müller-Karger et al., 1990; Aiken et al., 1995; Morel, 1997; O’Reilly et al., 1998, 2000). Since early 2018, the CMEMS GlobColour chlorophyll a product has been based on the merging of three algorithms: – the CI approach (Hu et al., 2012) for oligotrophic water (70 % to 80 % of ocean), – the common approach OCx (OC3, OC4 or OC4Me de- pending on the sensor) for mesotrophic water and – the OC5 algorithm (Gohin et al., 2002) for complex wa- ters, which is of specific interest for end users who man- age complex waters along the coastal zone. It should be noted that the OC5 algorithm is based on a lookup table implementation that handles both complex and mesotrophic waters (see Sect. 2.1) The work presented here highlights the conceptual advan- tage of the CMEMS Copernicus GlobColour processor with regards to the flagging and merging of sensors. In the follow- ing sections, results are described and illustrated with com- parison to the OC-CCI products. The comparison between GlobColour and OC-CCI is es- pecially relevant as the same chlorophyll a algorithms (CI and OC5) are used by the two initiatives: this implies that the differences in performance mainly result from the merging strategies and flagging schemes. Figure 1. Swath of the different sensors used at present by CMEMS for (a) MODIS Aqua, (b) VIIRS-NPP and (c) OLCI-S3A. In prac- 2 Methods tice the effective swath coverage is reduced, which is mainly due to clouds or sun glint effects. Source: http://octac.acri.fr (last access: 2.1 Merging approach 15 January 2019). At present, the CMEMS GlobColour-merged chlorophyll a product relies on the following sensors: SeaWiFS (1997– All of the sensors used observe the Earth along a helio- 2010), MERIS (2002–2012), MODIS Aqua (2002–present), synchronous orbit. Figure 1 displays the coverage of a single VIIRS-NPP (2012–present) and OLCI-S3A (2016–present). sensor that is unable to provide the full Earth coverage over a The long time series from 1997 to the present relies on dif- single day (Maritorena et al., 2015). VIIRS provides a larger ferent sensors, observing the Earth at different spectral bands swath than the other sensors, but the coverage is incomplete (and different bandwidths), with different acquisition times due to sun glint. (so different atmospheric and sun conditions), and with dif- When more than one sensor is available for the same pe- ferent spatial resolutions from about 300 m to 1 km at nadir riod it is possible to take advantage of their complementarity (larger on the swath border). The main characteristics of the and redundancy for a number of benefits. For instance, as sensors/bands used for CMEMS are summarised in Table 1. they may record the same spot at different times of the day, VIIRS-NOOA20 and OLCI-S3B will be ingested in the op- morning haze may impact one sensor but not another (Toole erational products in 2019. et al., 2000). It should be noted that the global chlorophyll a product To compute a multi-sensor chlorophyll a product, two is provided at a 4 km spatial resolution at present, but the main merging approaches exist: objective in the coming years is to provide (at least along the coast) a chlorophyll a product at a 300 m resolution. – The first approach (used by OC-CCI) is based on merged reflectance of remote-sensing (RRS) computed Ocean Sci., 15, 819–830, 2019 www.ocean-sci.net/15/819/2019/ P. Garnesson et al.: The CMEMS GlobColour chlorophyll a product Based on satellite observation 821 Table 1. Main characteristics of sensors/bands used for CMEMS (VIIRS-JPPS1 and OLCI-S3B will be used by the GlobColour processor in the framework of the CMEMS release scheduled in July 2019). Sensor RRS wavelengths (nm) Spatial resolution Swath Equator Period at nadir (km) width (km) crossing time SeaWiFS 412, 443, 490, 510, 555 and 670 1 and 4 1502 12:20 1997–2010 MERIS 413, 443, 490, 510, 560, 620, 1 and 0.3 1150 10:00 2002–2012 667, 681 and 709 MODIS Aqua 412, 443, 469, 488, 531, 547, 1 2330 13:30 2002–present 555, 645, 667 and, 678 VIIRS-NPP 410, 443, 486, 551 and 671 1 3040 10:30 2012–present OLCI S3A 400, 412, 442, 490, 510, 560, 1.2 and 0.3 1270 10:00 2016–present 620, 665, 674, 681 and 709 VIIRS-JPPS1/NOAA20 411, 445, 489, 556 and 667 0.75 3040 09:50 December 2017– present OLCI S3B 400, 412, 442, 490, 510, 560, 1.2 and 0.3 1270 10:00 2018–present 620, 665, 674, 681 and 709 in a prior step and then used to derive chlorophyll a. chlorophyll a, the input level 2 reflectance and input flags are Based on a band shifting and bias correction approach, used (level 2 provides flags/indicators regarding the quality merged RRS for the standard SeaWiFS wavelengths of the reflectance on a pixel basis). For instance, a pixel can (412, 443, 490, 510, 555 and 670 nm) are provided. The be impacted by a sun glint effect. In such cases reflectance blended chlorophyll a algorithm used in the OC-CCI data are available, but agencies recommend not to use them v3.1 release attempts to weight the outputs of the best for further processing. Each space agency publishes flagging performing algorithms based on the water types present. data based on its own strategy, which has been designed to guarantee the quality of its products. The drawback is that for – Conversely, in a second approach (used by CMEMS complex water (especially along coastal area), a large propor- GlobColour), chlorophyll a is computed in an initial- tion of the data is flagged, resulting in level 3 products with isation step for each sensor using its specific charac- limited coverage.
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